Surveillance control systems that control the surveillance of building facilities and plant facilities connect various communications equipment that has functions, such as information acquisition functions and control functions, as nodes via communications networks and carry out surveillance control of individual pieces of equipment through a central monitor based on information from these nodes. Ethernets are used as the communications networks in these surveillance control systems.
When a plurality of nodes is connected to an Ethernet, the basis is a star-wired system where each node is connected to a hub or a switch. While this star-wired system is suitable for a comparatively small scale office environment, it is not always suitable for large-scale facilities such as building facilities or plant facilities. A reason for this is that with a star-wired system, it is necessary to connect the hub or switch and each node via its own individual wiring. When a wide range of nodes are installed, the wiring connecting the nodes becomes complicated, and there is an increase in the work burden for wiring construction and maintenance.
Ring type Ethernets, which make connections with wiring systems across all nodes, have been proposed for this kind of Ethernet. System redundancy may be achieved for these ring type Ethernets using the Spanning Tree Protocol (STP/IEEE 802.1D) function, which avoids communications errors using a ring topography present in the communication path, an improved Rapid STP (RSTP/IEEE 802.1W) function or the like network control function. FIG. 9 is an example of the constitution of a typical ring type Ethernet system. Here, a plurality of nodes N is connected in a ring L via a ring type Ethernet switch. Normally, one root node R is selected out of the nodes connected to the ring by the network control functions for RSTP, STP and the like built into Ethernet switches, and the communications path that is currently used for the tree topology is established based on the ring cost between nodes using an exchange of network control information called Bridge Protocol Data Units (BPDUs) between this root node R and the other nodes.
At this time, the unnecessary communications paths other than the communications path that is currently used are established as backup communications paths for times when there is a failure by blocking the port for the node in question. In the example in FIG. 9, going around to the left and going around to the right may be considered as paths from the root node R to node N1. When the cost going around to the left is less than the cost going around to the right at this time, the path going around to the left is selected as the communications path that is currently used. Therefore, the path away from the node N1 to the node N2 becomes an unnecessary path, and blocking is done at either one of node N1 or node N2, which are at the endpoints of this unnecessary path. Therefore, the original ring L formed from the ring topology is changed to a tree topology formed from two branch paths from the root node R to node N1 and node N2.
Therefore, even if a network is physically formed in a ring topology, the generation of data loops is avoided. In addition, when the BPDUs periodically sent by the root node R to any node cannot be received, it is determined that a failure has arisen in the path between the root node R and the node in question. In such a case, a reconstruction request is sent from the node in question in the opposite direction of the root node R. In response to receiving this reconstruction request, the node that is being blocked cancels the blocking in question. In this manner, the backup communications path that had been blocked is used, and there is reconstruction for a new communications path.
Therefore, when there is a failure at a point P in the example in FIG. 9, a reconstruction request is sent by node N3, and the blocking from node N1 at point B is canceled. There is reconstruction for a new pathway from the root node R to node N3.
When a large-scale Ethernet that is used in a building facility or plant facility is achieved with a single ring using this ring type Ethernet, all nodes share a single ring, so the reliability as a system is reduced.
As technology for solving this problem, a method where the various nodes are divided into a plurality of sub-rings and connected, a typical switching hub used and these sub-rings connected to each other has been proposed. An example of this is disclosed in Japanese Published Unexamined Patent Application No. 2006-174422, which is hereby incorporated by reference in its entirety. Therefore, since the risk is distributed over the various sub-hubs, there is an improvement in reliability as a system.
Since, the physically independent sub-rings are connected through typical switching hubs in this conventional technology, not only user data but also network control data is reciprocally communicable among the sub-rings. In other words, not only are there Ethernet communications, but also a single domain is formed in the network control.
Therefore, there is a problem in that, according to the conventional technology described above, a failure arising in any sub-ring is propagated to the other sub-rings, and since communications path reconstruction operations are executed by the network control function even in the normal sub-rings, there is a temporary failure in the Ethernet communications between nodes connected to the normal sub-rings.
FIG. 10 is an example of the constitution of a ring type Ethernet according to the conventional technology. Here, the constitution has three sub-rings L1 through L3, and each of them is connected to the switching hub. This switching hub corresponds to the RSTP, STP or other network control function built into the nodes, and the BPDUs and reconstruction requests used by this network control function have functions that are transferred to the sub-rings L1 through L3.
Therefore, the sub-rings L1 through L3 are seen as a single ring in the network control function, and the BPDUs sent from the root node R of the sub-ring L1 are also transferred to the other sub-rings L2 and L3.
Thus, for example, when a failure occurs at point P on ring L1, a reconstruction request is sent from node N1 to node N2, and the blocking at point B between node N1 and node N2 is canceled. Because of this, the backup communications path joining node N1 and node N2 that had been blocked is used, and there is reconstruction of a new communications path from the root node R to the node N1.
Since there is reconstruction for the new communications path at this time, reconstruction operations are carried out for the various nodes connected to the sub-rings L2 and L3 other than the sub-ring L1 where the failure occurred; therefore, there is a temporary failure in the Ethernet communications among the nodes connected to the normal sub-rings.